Chemical Abundances in 12 Red Giants of the Large Magellanic Cloud from High-Resolution Infrared Spectroscopy

Abstract

High-resolution infrared spectra ({λ}/{Δ}{λ}=50,000) havebeen obtained for 12 red giant members of the Large Magellanic Cloud(LMC) with the Gemini South 8.3 m telescope and Phoenix spectrometer.Two wavelength regions, at 15540 and 23400 {Å}, were observed.Quantitative chemical abundances of carbon (both ^{12}C and^{13}C), nitrogen, and oxygen were derived from molecular linesof CO, CN, and OH, while sodium, scandium, titanium, and iron abundanceswere obtained from neutral atomic lines. The 12 LMC red giants span ametallicity range from [Fe/H]=-1.1 to [Fe/H]=-0.3. It is found thatvalues for both [Na/Fe] and [Ti/Fe] in the LMC giants fall below theircorresponding Galactic values (at these same [Fe/H] abundances) by about~0.1-0.5 dex; this effect is similar to abundance patterns found in thefew dwarf spheroidal galaxies with published abundances. The program redgiants all show evidence of first dredge-up mixing of material exposedto the CN cycle, that is, low ^{12}C/^{13}C ratios andlower ^{12}C with higher ^{14}N abundances. The carbonand nitrogen trends are similar to what is observed in samples ofGalactic red giants, although the LMC red giants seem to show smaller^{12}C/^{13}C ratios for a given stellar mass. Thisrelatively small difference in the carbon isotope ratios between LMC andGalactic red giants could be due to increased extra mixing in stars oflower metallicity, as suggested previously in the literature.Comparisons of the oxygen-to-iron ratios in the LMC and the Galaxyindicate that the trend of [O/Fe] versus [Fe/H] in the LMC falls about0.2 dex below the Galactic trend. Such an offset can be modeled as dueto an overall lower rate of supernovae per unit mass in the LMC relativeto the Galaxy, as well as a slightly lower ratio of supernovae of TypeII to supernovae of Type Ia. Based on observations obtained at theGemini Observatory, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under a cooperative agreement with theNSF on behalf of the Gemini partnership: the National Science Foundation(US), the Particle Physics and Astronomy Research Council (UK), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil), and CONICET (Argentina).